In its simplest form, a hydraulic power drive or hydrostatic transmission consists of the following components:
(a) a prime mover, the source of power--a gasoline engine, electric motor, etc.
(b) a hydraulic pump, normally a variable displacement pump, which is driven by the prime mover and which produces a flow of pressurized hydraulic fluid;
(c) a hydraulic motor which is driven by the hydraulic pump;
(d) a means for controlling the direction of flow and magnitude of flow out of the pump; and
(e) mechanical shafting and gearing to convey the output of the hydraulic motor to the load. The hydraulic pump is normally driven at a constant speed by the prime mover; if it is a variable displacement pump, it delivers hydraulic fluid at a variable rate to the hydraulic motor. Thus, by controlling the output of the hydraulic pump, the direction and speed of the hydraulic motor, and hence direction and speed of the load can be controlled.
In many hydraulic drive systems or hydrostatic transmissions, a mass or load is accelerated and brought to speed by pumping fluid in one direction from the hydraulic pump to the hydraulic motor and decelerated or braked by attempting to reverse the flow of fluid from the hydraulic pump to the hydraulic motor (e.g. dynamic braking). This method of controlling the acceleration and deceleration of an inertial load is convenient, stable, well-proven and precise. If the reversing feature of the hydraulic pump is used in an ordinary hydrostatic drive, the same hydraulic pressure is normally used to both accelerate and decelerate the load.
In the case of an axial piston hydraulic pump the amount of flow and the direction of flow is determined by the angle of the tilt plate or swash plate. These axial piston pumps are often referred to as variable delivery or variable displacement hydraulic pumps. When the swash plate is at right angles to the drive shaft (i.e. zero tilt) and the drive shaft is rotating, the pistons which pump the fluid do not reciprocate; therefore, no pumping action takes place. When the swash plate is tilted from the right angle position, the pistons reciprocate and hydraulic fluid is pumped. The design and operation of axial piston hydraulic motors is very similar to the design and operation of an axial piston hydraulic pump. It well known that some hydraulic pumps can be used as hydraulic motors with little or no modification.
The output speed and torque of a hydraulic motor depend upon the power input in terms of differential pressure and the rate of flow. The relationship between output speed vs. input volume and output torque vs. input differential pressure depends on displacement of the motor. In particular, the output torque is proportional to the input differential pressure and the flow rate and inversely proportional to the speed (RPM). This follows from the fact that in a prime mover hydraulic pump system combination, the torque of the input shaft is proportional to the hydraulic pressure and the flow through the system. Therefore, if system pressure is limited, shaft torque or input torque will also be limited, for a given pump displacement.
Fluid motors may be of the fixed displacement or variable displacement type. The fixed displacement provides constant torque and variable speed for a given differential pressure. The speed is varied by controlling the amount of input flow. The variable displacement motor is constructed in a manner which permits the working relationship of the internal parts to be varied so as to change the displacement. This provides variable torque and variable speed. With input flow and operating pressure remaining constant, the relationship between the torque and the speed can be varied to meet load requirements by varying the displacement. The majority of motors used in fluid power systems are the fixed displacement types. In most fluid power systems, the motor is required to provide actuation in either direction. This is, of course, achieved by changing the angle of the swash plate in the variable displacement hydraulic pump.
Because of the similarity in the operation and construction of the hydraulic pump and hydraulic motor, particularly a variable displacement hydraulic pump of the axial piston variety and an axial piston hydraulic motor, a hydraulic motor acts as a hydraulic pump when the hydraulic pump has its displacement reduced (by decreasing the angle of the swash plate) to a volume less than the volume passing through the motor and the load connected to the output shaft of the hydraulic motor is in motion. Effectively, the inertia of the load drives the hydraulic motor in such a way that it "pumps" hydraulic fluid.
However, most prime movers do not have a retarding torque characteristic equal to their driving torque characteristic. In other words, the prime mover accelerates a load at a different rate than it decelerates a load. Consequently, if a load in motion is decelerated by the application of decelerating pressure, the magnitude of which is equal to the pressure previously used to accelerate that load, the hydraulic pump and the associated prime mover can overspeed. Overspeeding the hydraulic pump or the prime mover can cause damage or excessive wear to either or both of the components. Thus, if overspeeding is to be avoided, a lower pressure should be used to decelerate the moving load. Although the rate of deceleration is reduced by using a lower hydraulic pressure, operationally this is of no significance since most machines also have mechanical brakes to slow or stop the moving load. These mechanical brakes are normally used in conjunction with the retarding feature of the variable displacement hydraulic pump.